This invention relates to a source voltage control switching circuit for maintaining the source voltages to be stable at the light-load powering time of BS (Direct Broadcasting Satellite) timer recording mode and at the maximum load powering time of television mode.
Recently, most television receivers are of the BS-tuner-built-in type. Thus, a converter switching power source for the BS built-in type has been developed so far.
The source voltage control switching circuit used in the above switching power source will be described with reference to the accompanying drawings.
FIG. 5 shows a conventional source voltage control switching circuit.
Referring to FIG. 5, there are shown an AC rectifying circuit 1, a power control integrated circuit (hereinafter, referred to as the power control IC) 2, a rectifying capacitor 3, a power output MOSFET 4, a power transformer 5, a voltage control feedback photocoupler 6, a start resistor 7, a rectifying diode 8, a rectifying capacitor 9, a 18-V control constant voltage diode 10, rectifying diodes 11, 12, a relay 13 for turning a television source voltage 140 V on and off, rectifying capacitors 14, 15, an error amplifying shunt regulator 16, a relay drive transistor 17, a switching transistor 18 for turning the 18-V control constant voltage diode on and off, a switching transistor 19 for turning the 140-V control error amplifying shunt regulator on and off, and diodes 20, 21 and resistors 22, 23, 24, 25, 26, 27.
The operation of this conventional source voltage control switching circuit will be mentioned below.
When the AC power supply is turned on, the control switching circuit is started through the start resistor 7 so that voltages are induced in the secondary windings of the transformer 5. At the same time, a high-level signal in the television mode is supplied to the bases of the transistors 17 and 19 so that the transistor 17 becomes conductive to energize the relay 13. Thus, upon television reception, the television source voltage 140 V and the BS source voltage 18 V are supplied to the load side. At this time, the transistor 19 is also conductive, thus grounding the 3-pin of the error amplifying shunt regulator 16. Thus, the secondary output voltages are controlled by use of the television source voltage 140 V. In other words, the voltage change fed from the 140-V line through the resistor 27 is converted by the error amplifying shunt regulator 16 into a current change, which is then fed back to the power control IC 2 through the photocoupler 6 so that the source voltages can be stabilized.
In the BS timer recording mode, the voltage to the bases of the transistors 19, 17 becomes low, and thus the transistors 19, 17 are nonconductive. "BS timer recording mode" is a mode in which connection of a relay circuit is cut, the television receiver circuit does not operate and only BS broadcasting signal is receiving condition. Therefore, the relay 13 is deenergized so that the television source voltage is not supplied to the load side, and thus only the BS source voltage is supplied to the load side. At this time, since the transistor 19 is nonconductive, the error amplifying shunt regulator 16 is inoperative. Thus, the secondary output voltages are controlled through the BS source voltage 18 V, the resistor 22, the transistor 18 and the constant voltage diode 10. In other words, the voltage change on the 18-V line is converted into a current change, which is then fed back to the power control IC 2 through the photocoupler 6 so that the source voltages can be stabilized. At this time, the BS source voltage value depends on the voltage value of the constant voltage diode.
Another conventional example will be described below.
FIG. 6 is a circuit diagram of the converter switching power source for the BS tuner built-in television receiver. Referring to FIG. 6, the output end of an AC rectifying circuit 31 is connected to one end of a primary winding 32a of a power transformer 32, to one end of a start resistor 33, and to one end of a rectifying capacitor 34 of which the other end is grounded. The other end of the start resistor 33 is connected (1) to the input terminal of a power control integrated circuit (hereinafter, referred to as the power control IC) 35, (2) to the collector of the phototransistor of a photocoupler 36, (3) to one end of a rectifying capacitor 37 with the other end grounded, and (4) to the cathode of a rectifying diode 38. The emitter of the phototransistor of the photocoupler 36 is connected to the control terminal of the power control IC 35. The anode of the rectifying diode 38 is grounded through the secondary winding, 32b of the power transformer 32. The output terminal of the power control IC 35 is connected to the base of a power output MOS transistor (hereinafter, referred to as MOSFET) 39. The drain of the MOSFET 39 is connected to the other end of the primary winding 32a of the power transformer 32, and the source of the MOSFET 39 is grounded.
One end of a secondary winding 32c of the power transformer 32 of which the other end is grounded is connected to the anode of a rectifying diode 40. The cathode of the rectifying diode 40 is connected to one end of a rectifying capacitor 41 of which the other end is grounded, and to the BS circuit, supplying+48 V to the BS circuit. In addition, one end of a secondary winding 32d of the power transformer 32 of which the other end is grounded is connected to the anode of a rectifying diode 42. The cathode of the rectifying diode 42 is connected to one end of a rectifying capacitor 43 of which the other end is grounded, and to the television circuit through relay contacts 44a of a relay 44, thus supplying+130 V to the television circuit. Also, the collector of a switching transistor 46, of which the emitter is grounded and of which the base is supplied with a low level signal (hereinafter, abbreviated as the L-signal) through a resistor 45 in the television receiving mode and with a high level signal (hereinafter, abbreviated the H-signal) through the resistor 45 in the BS timer recording mode, is connected to the cathode of the rectifying diode 42 through a dummy load resistor 47. An error amplifying shunt regulator 48 has a terminal 48a connected to the cathode of the rectifying diode 42 through a volume control 49, a terminal 48b connected to the cathode of the photodiode of the photocoupler 36 of which the anode is connected to+B power source, and a terminal 48c grounded. Moreover, a relay drive transistor 51 has its emitter grounded, its base supplied with the H-signal through a resistor 50 in the television receiving mode and with the L-signal through the resistor 50 in the BS timer recording mode, and its collector connected through a relay winding 44b of the relay 44 to+12 V source voltage.
The operation of the above construction will be mentioned. First, when the AC power supply is turned on, a source voltage is supplied from the AC rectifying circuit 31 to the input terminal of the power control IC 35 through the start resistor 33. The power control IC 35 produces a signal at its output terminal, causing the MOSFET 39 to perform switching operation. Thus, the power source is actuated so that voltages are induced in the secondary windings 32b, 32c and 32d of the power transformer 32.
In the television receiving mode in which a picture is displayed on the cathode ray tube, as soon as the voltages are induced in the secondary windings 32b, 32c and 32d of the power transformer 2, the H-signal is supplied to the base of the transistor 51, making the transistor 51 conductive so that the relay 44 is energized. Thus, the relay contacts 44a are closed, allowing the television circuit to be supplied with+130 V from the secondary winding 32d so that a picture can be displayed on the cathode ray tube. In addition, the BS source voltage+48 V is supplied from the secondary winding 32c to the BS receiving circuit and so on. At this time, the error amplifying shunt regulator 48 converts the voltage change of the television source voltage+130 V line into a current change, and feeds it through the photocoupler 36 back to the control terminal of the power control IC 35 so that the source voltage of the television power source+130 V line can be stabilized. The base of the transistor 46, in the television receiving mode, is supplied with the L-signal so that the transistor 46 is not conductive.
Then, in the BS timer recording mode in which a BS program is received by only operating the BS receiving circuit and recorded on a VTR, the base of the transistor 51 is supplied with the L-signal so that the transistor 51 is nonconductive. Thus, the relay 44 is not energized so that the relay contacts 44a are opened. Therefore, the television source voltage is not supplied to the load side, or the television circuit, but only the BS source voltage is supplied to the BS receiving circuit. At this time, the base of the transistor 46 is supplied with the H-signal in the BS timer recording mode so that the transistor 46 is conductive, thus connecting the dummy resistor 47 between the television power source+130 V line and ground. The current (several tens of mA) flowing in the dummy load resistor 47, preventing the no-load state. Thus, the source voltage stabilization at the light-load powering time can be achieved by use of the error amplifying shunt regulator 48.